The broad goal of this research proposal is to elucidate the basic mechanisms underlying radiation-induced fatigue in patients with localized breast or prostate cancer, focusing on psychological and immunological factors that may contribute to fatigue during and after treatment. Preliminary studies conducted by our group have identified psychological and immune parameters associated with fatigue in breast cancer survivors, including several markers of immune activation. Pilot testing will first be conducted to determine which of these immune parameters are influenced by radiation treatment. The main phase of the study will involve a prospective, longitudinal examination of 150 breast and prostate cancer patients receiving external beam radiation therapy. Subjects will be recruited from the UCLA Radiation Oncology Clinic and will be assessed psychologically and immunologically before treatment onset, at biweekly time points during treatment, and at three longer-term follow-ups. The specific aims of the study are: 1) to determine whether changes in the immune system induced by radiation therapy are associated with changes in fatigue, 2) to evaluate the role of psychosocial factors as predictors and correlates of radiation-induced fatigue, 3) to evaluate the effects of irradiated site and treatment volume on fatigue, and 4) to identify individuals whose fatigue does not remit following treatment and determine the correlates of acute vs. more enduring fatigue. Information gained from this project will advance our understanding of radiation-induced fatigue, thereby paving the way for the development of interventions to help manage and reduce this problem and improve quality of life for cancer patients receiving radiation therapy. The proposed research project will enhance the candidate's existing research skills in health psychology and psychoneuroimmunology and promote the development of new skills necessary for an academic career in cancer prevention and control. Together with a targeted program of didactic instruction, this project will prepare the candidate to become a fully independent behavioral scientist in the field of cancer prevention and control research.